So-called collimated sputtering and long-throw sputtering have been used for coating moderate aspect ratio holes. Ionized Physical Vapor Deposition, IPVD, has been used more recently to deposit films in holes. In the IPVD method a flux of ionized metal atoms is used. Such flux of positively charged metal ions is accelerated in the gap between the plasma and the substrate, e.g. a silicon wafer which has a negative bias with respect to the plasma. As the electric field is perpendicular to the substrate as to a silicon wafer surface, this results in a superior bottom coverage of high aspect ratio holes. There are various ways how to achieve high ionization fraction of metal for IPVD. One way is known from the U.S. Pat. No. 6,352,629. Before discussing this prior art and proceeding to the present invention some definitions shall be established:
1. Magnetron Magnetic Field Pattern
As exemplified in FIG. 1 a magnetron magnetic field pattern as established along a target surface 3 of a target 1 comprises, seen towards the target surface 3, a pattern of magnetic field FM which forms a closed loop. In a cross-sectional view onto the target the magnetron magnetic field pattern FM is tunnel-shaped with magnetic field arcing from an outer area Ao of one magnetic polarity to an adjacent inner area Ai with the other magnetic polarity. The magnetic flux out of the outer area Ao which forms a substantially closed loop is substantially equal to the magnetic flux at the second, inner area Ai except the signum.
Thereby, we define the outer area Ao as confined by a closed loop locus line L′ which is defined by the projection (dashed lines) of the locus L along the magnetic field pattern FM along which the component of magnetic field perpendicular to the target surface 3 is zero.
Further, whenever the present invention is applied with etching the target surface 3 is of a non-sputtered material. For the preferred application of the present invention, i.e. for sputter-coating the target surface 3, the target surface is of a material to be sputtered and is therefore a sputtering surface.
2. Magnetron Magnetic Field with Unbalanced Component Pattern
The magnetron magnetic field pattern becomes unbalanced if, departing from the balanced configuration as of (1), the magnetic flux along one of the inner Ai and of the outer—Ao—areas is increased relative to such flux at the other area. In FIG. 1 there is schematically shown the generation of the magnetron magnetic field pattern FM and, additionally, of an unbalanced field pattern FU. Along the target 1 and adjacent the target surface opposite to the target surface 3 there is provided a magnet arrangement with an inner magnet subarrangement 5 and a second outer magnet subarrangement 7. The surface of first subarrangement 5 facing the target 1 is of one magnet polarity, S, whereas the surface of the outer subarrangement 7 facing target 1 has the second magnet polarity, N. Between the two magnet subarrangements there is formed the magnetron field pattern FM, whereby the magnetic flux at the surfaces of the two magnet subarrangements 7 and 5 is substantially equal.
Whereas in FIG. 1 the field pattern FM is generated by means of magnet subarrangements 5 and 7, which respectively have magnetic dipoles oriented perpendicularly to the target surface 3, this field pattern FM may also be generated by respective magnet arrangements with magnetic dipoles substantially parallel to the target surface 3, one pole providing for the magnetic flux at the inner area Ai, the other magnetic pole for the magnetic flux at the outer area Ao.
The magnetron field pattern becomes unbalanced if according to FIG. 1 the magnetic flux at one of the respective surfaces with the subarrangements 5 and 7, according to FIG. 1 at the outer area Ao, is significantly increased. There occurs, compared with the magnetron field pattern FM, a considerable amount of magnetic flux FU with long range. In FIG. 1 as an example there is shown a centered circular arrangement of the two subarrangements 5 and 7 with respect to a loop central axis AL.
The unbalanced field pattern FU is evenly distributed along the outer magnet subarrangement 7.
Such known unbalanced field pattern FU is thus the result of increasing the magnetic flux e.g. at the outer area Ao with a homogeneous increase of magnetic flux density along a loop of that area Ao. In view of the present invention we call such unbalanced field pattern FU as of FIG. 1 a symmetrically unbalanced field pattern.
Turning to the U.S. Pat. No. 6,352,629 it may be seen that there is provided a magnet arrangement which generates a symmetrically unbalanced field pattern as was explained with the help of FIG. 1, which is moved around an axis offset from the loop central axis AL of the symmetrically unbalanced circular magnetron. There is provided a DC coil which is wrapped around the space between the target and the substrate being sputter-coated so as to generate an axial magnetic field guiding metal ions towards the substrate. The target area which is covered by the symmetrically unbalanced magnetron field pattern is considerably smaller than the overall sputtering surface.
As a symmetrically unbalanced magnetron as shown in FIG. 1 generates an extremely focused plasma on the loop central axis, the ion density at the substrate is strongly inhomogeneous.